FIG. 1 illustrates radio resources of a downlink signal in a Long Term Evolution (LTE) system. In FIG. 1, the horizontal axis represents a time axis, and the vertical axis represents a frequency axis.
In the LTE system, Orthogonal Frequency Division Multiplexing (OFDM) is used in downlink communications. Specifically, one radio frame has ms, and data are communicated in one millisecond called a subframe. One subframe has two slots (first half and second half slots), and each slot has seven OFDM symbols.
The first three OFDM symbols in the first half slot are assigned (allocated) to a control channel called a Physical Downlink Control Channel (PDCCH). Further, Downlink Control Information (DCI) is assigned to the Physical Downlink Control Channel (PDCCH). The Downlink Control Information (DCI) reports assignment (allocation) information of resource blocks (RB) assigned to each user equipment (UE) and RB assignment information in uplink to each user equipment.
Herein, the “RB” refers to the minimum unit assigned to user data and having twelve subcarriers in each slot. A set of the OFDM symbols excluding the OFDM symbols for the Physical Downlink Control Channel (PDCCH) is called a Physical Downlink Shared Channel (PDSCH). The number of the symbols of the Physical Downlink Control Channel (PDCCH) is determined by the system.
FIG. 2 illustrates radio resources of an uplink signal in the LTE system. In FIG. 2, the horizontal axis represents a time axis, and the vertical axis represents a frequency axis.
The radio frame, the subframe, the slot and the RB in the uplink signal are the same as those in the downlink signal.
The radio resources in the uplink signal are assigned to a Physical Uplink Control Channel (PUCCH) and a Physical Uplink Shared Channel (PUSCH) The Physical Uplink Control Channel (PUCCH) is used to report Acknowledgement/Negative Acknowledgement (ACK/NACK) in response to user data in the downlink signal and channel quality indicator (CQI) at the user terminal (or User Equipment (UE)). Further, user data are assigned to the Physical Uplink Shared Channel (PUSCH).
The Physical Uplink Control Channel (PUCCH) is assigned at both ends of the system bandwidth by the RB. The area of the resource blocks of the Physical Uplink Control Channel (PUCCH) is determined by the system.
The transmission timing of the uplink signal is corrected (adjusted) by using a Timing Advance (TA) Command. Here, the correction (adjustment) of the transmission timing of the uplink signal refers to a process of correcting (adjusting) the transmission timing of the uplink signal performed by the user terminal based on instructions from the base station (eNB: eNodeB) so that the base station may receive the Physical Uplink Shared Channel (PUSCH) and the Physical Uplink Control Channel (PUCCH) and a Sounding Reference Signal (SRS) at appropriate timings at the receiving end of the base station.
The base station calculates an offset of the reception timing of the uplink radio frame from the user terminal by using the transmission timing of the downlink radio frame as a reference. When the offset occurs (is required), the base station issues the timing advance command to instruct the user terminal to correct the transmission timing of the uplink signal. The timing correction is performed by a 0.52 μs step. A value “0x1F” of the reference of the timing advance command is used as a reference. Namely, the value “0x1F” of the reference of the timing advance command indicates that no correction of the transmission timing is necessary.
There has been known a method of correcting the transmission timing of the uplink signal (see, for example, 3GPP TS36.321 V10.1.0, “5.2 Maintenance of Uplink Time Alignment” 2011-03).
FIG. 3 illustrates an example sequence diagram in data communication between the base station (eNB) and the user terminal (UE). More specifically, FIG. 3 illustrates a sequence where the base station stops the transmission of the Timing Advance Command.
Before starting the data communication, the base station and the user terminal execute a Random Access Procedure (Initial Access). In FIG. 3, a process in steps S31 through S35 is called the Random Access Procedure (Initial Access).
The user terminal transmits a Random Access Preamble to the base station (step S31).
Upon detecting (receiving) the Random Access Preamble, the base station transmits a Random Access Response to the user terminal (step S32). The Random Access Response includes uplink RB assignment information and Temporary C-RNTI. The RB assignment information is necessary for the transmission of an Uplink-Shared Channel (UL-SCH) by the user terminal (step S32).
Upon receiving the Random Access Response, the user terminal transmits an RRC connection Request (RRC message) to the base station (step S33).
After the reception of the RRC connection Request, the base station transmits an RRC Connection Setup (RRC message) to the user terminal (step S34).
After the reception of the RRC connection Setup, the user terminal transmits an RRC Connection Setup Complete (RRC message) to the base station (step S35).
After the completion of the Random Access Procedure (Initial Access), the base station uses an RRC Connection Reconfiguration (RRC message) to report the setting values of the Physical Uplink Control Channel (PUCCH) and the Sounding Reference Signal (SRS) to the user terminal (step S36).
The user terminal transmits an RRC Connection Reconfiguration Complete to the base station as the response to the setting values. After that, U-plane communications are performed.
Further, after the completion of the Random Access Procedure (Initial Access), the user terminal monitors the Timing Advance (TA) Command from the base station. Depending on whether the user terminal receives the Timing Advance (TA) Command, the user terminal transmits the ACK or the NACK (step S39). Upon the reception of the Timing Advance (TA) Command, the user terminal starts a Time Alignment Timer (again).
The base station stops the transmission of the Timing Advance (TA) Command (step S41). When the Time Alignment Timer is terminated, the user terminal releases the Physical Uplink Control Channel (PUCCH) and the Sounding Reference Signal (SRS) (step S42). To transmit the Physical Uplink Control Channel (PUCCH) and the Sounding Reference Signal (SRS) by the user terminal, it is necessary to execute the Random Access Procedure (Initial Access). The value of the Time Alignment Timer is determined by the system.